1. Field of the Invention
The present invention broadly relates to continuous casting of a steel which contains slag reducing metal elements such as Al, Ti, REM (Rare Earth Metal) or the like and, more particularly, to a mold powder which can effectively be used for improving lubrication between the steel and the mold in order to avoid problems such as deterioration of the surface quality of the slab and break out of the slab which otherwise may occur due to inferior lubrication.
2. Description of the Related Art
Hitherto, in continuous casting of a steel containing 0.010 wt % or more Al, reducing reactions take place in accordance with the following formulae (1) and (2) between Al component in the molten steel and low-grade oxides such as SiO.sub.2, MnO and so forth contained in the powder which is used for providing lubrication between the mold wall and the slab and for covering the surface of the molten steel. At the same time, Al.sub.2 O.sub.3 in the molten steel, which is a deoxidation product or a product of secondary oxidation of the molten steel, floats in the molten steel in the mold so as to be trapped by the powder. ##STR1##
Consequently, the molten powder composition is changed to enhance the tendency of generation of 2CaO.Al.sub.2 O.sub.3.SiO.sub.2 (gehlenite), which undesirably promotes generation of slag and impairs lubrication.
Under this circumstance, methods are being developed which can avoid the above-mentioned change in the molten powder composition despite the increase in the Al.sub.2 O.sub.3 content in the powder, thereby ensuring stable casting while preventing deterioration of the surface quality of the product slab.
For instance, Japanese Patent Laid-Open No. 61-186155 discloses a powder which contains BaO and Li.sub.2 O, with the composition adjusted to meet the conditions of BaO+CaO+SiO.sub.2 =60 to 80 wt % and CaO/SiO.sub.2 =0.30 to 0.60 wt %.
Japanese Patent Laid-Open No. 63-10052 discloses a powder having a composition meeting the condition of CaO/SiO.sub.2 =0.6 to 0.8, melting point of 800.degree. to 1000.degree. C. and viscosity of 1300.degree. C.&lt;1.5 poise. Japanese Patent Laid-Open No. 3-77753 proposes a powder containing 10 to 20 wt % of SiO.sub.2 and not more than 15 wt % of Al.sub.2 O.sub.3, the SiO.sub.2 and Al.sub.2 O.sub.3 contents being determined to meet a specific condition.
All these proposed powders, however, have SiO.sub.2 content ranging from 10 to 40 wt % and, therefore, cannot completely eliminate the above-described problems. Thus, when these known powders are used, thermit reactions take place in accordance with the aforesaid formulae (1) and (2) to generate heat, thus posing the following problems.
(i) Generation of bleed (small scale of breakage of solidification shell) along oscillation mark due to delay of solidification at the meniscus. PA1 (ii) Reduced Si is mixed in the molten steel to make the Si content of the molten steel fall out of the target range. At the same time, Al in the molten steel is changed to Al.sub.2 O.sub.3 to make the Al content fall out of the target range. Consequently, the desired composition of molten steel cannot be obtained. PA1 (iii) Flame is formed as a result of the thermit reaction, which causes danger in the casting operation, making it difficult to safely continue the casting.
Meanwhile, Japanese Patent Publication No. 63-56019 proposes a powder in which SiO.sub.2 content is diminished as much as possible so as to avoid the reaction in accordance with the aforesaid formula (1).
This powder has an increased CaO content of 40 to 60 wt % at the cost of the reduced SiO.sub.2 content. Therefore, the powder exhibits inferior fluidity, which impairs lubrication of the steel resulting in deterioration of the surface quality of the product slab.